In the oil and gas industry, after drilling a wellbore it is common practice to line the wellbore with one or more strings of pipe known in the industry as “casing,” and secure the casing in the wellbore with cement pumped into the wellbore annulus defined between the casing and the wall of the wellbore. In some cases, two or more strings of casing are concentrically positioned in the wellbore and cement is pumped between the casings and the wellbore annulus to secure the casings within the wellbore.
Good cement bonding characterization between the casing and the wellbore, and also the location and distribution of other classes of downhole materials and their characterization, is essential and particularly critical in the case of plug and abandonment operations. For instance, accurately characterizing the materials or substances disposed within the annulus, and determining their azimuthal and depth distributions throughout the wellbore may help an operator determine a preferred location to cut the casing so that upper portions of the casing may be pulled out of the wellbore. More particularly, determining the azimuthal and depth location of particular materials present within the annulus may help determine where the casing is relatively “free” or has little resistance to being extracted (pulled) from the well after being cut. It is also desirable to estimate the forces required to extract cut casing when portions of the casing are covered entirely or in part by solids and/or gelled materials that increase the friction existing between the casing and materials in the annulus. Additionally, it is desirable to estimate the presence of gas and/or lighter fluids that may pose a risk or hazard to operations performed during well intervention and abandonment activities.
Past methods to accomplish this include using data acquired from cement bond logging tools, such as omni-directional or sectored/segmented logging tools, ultrasonic measurement tools, and pad-mounted spectral density logging tools. Like the sectored/segmented cement bond logging tools, the pad-mounted spectral density logging tools acquire data only from a sector of the wellbore and do not acquire data from the entire circumference of the wellbore. Further, in deviated wellbores, the pad-mounted spectral density logging tools may only acquire data only from the lower side of the wellbore since the weighted measurement pad may become oriented in the downward direction due to gravity. It is therefore difficult to accurately determine the presence of certain substances, such as settled drilling fluid (“mud”) solids, in the wellbore annulus and in between the casings and thereby differentiate these substances from the cement present in the wellbore annulus and in between the casings.
Over a period of years from the initial completion of the well to the time of well abandonment, drilling fluids left in place in the wellbore annulus deteriorate and precipitate the suspended weighting materials, which often accumulate between concentric or overlapping layers of casing. These solids can act as a binding agent that makes it harder to extract cut casing above a cutting depth.
By relying on legacy acoustic and spectral density measurements, the identification of such solids is often partially inaccurate. This is because acoustic sensor readings for such solids fail to provide significant contrast to adjacent materials present in the wellbore annulus at a suitable level sufficient for identification purposes. This often results in the incorrect determination of the character of materials within the annulus and, therefore, a resulting miscalculation of optimal or feasible cutting forces required to extract the casing.